For example, a suspension substrate mounted with a device such as a magnetic head slider is known as a circuit board used for an HDD (a hard disk drive). The suspension substrate ordinarily has a fundamental structure such that a metal supporting substrate (such as SUS), an insulating layer (such as a polyimide resin) and a wiring layer (such as Cu) are laminated in this order; and ordinarily has, at one tip, a device mounting area which is mounted with the device, and has, at the other tip, a connecting area for connecting with another circuit board.
Also, it is known that the metal supporting substrate is typically so high in rigidity that part of the metal supporting substrate is removed for intending to achieve lower rigidity. On the other hand, the removal of part of the metal supporting substrate occasionally causes a problem resulting from lack of mechanical strength by contraries. Thus, a technique for reinforcing the metal supporting substrate by using an insulating material is known. For example, in Patent Literature 1, a suspension assembly having a specific dielectric limiter is disclosed. This technique relates to a limiter for preventing a flexure assembly from plastically deforming and being incapable of pivoting in a predetermined manner in the case where acceleration is applied to a head/slider of an HGA (a head gimbal assembly) by impact force and the flexure assembly for supporting the head/slider is displaced over a certain level, and relates to a dielectric limiter suitable for downsizing. The plastic deformation of the suspension assembly is occasionally caused resulting from not merely the impact on a magnetic disk device but also the handling of the HGA in a manufacturing process. However, in order to restrain such deformation of the dielectric limiter itself, the width of the limiter needs to be maintained at a predetermined size, whereby a problem that thinning is difficult occurs.
Also, the following technique is known as a technique for processing a polyimide resin used for an insulating layer. For example, in Patent Literature 2, it is described that a resist image for processing an insulating layer is formed to process the insulating layer by wet etching in accordance with the resist image. Also, in Patent Literature 3, it is described that only an area to be removed in a polyimide resin with a faster dissolution rate is selectively removed by the same etching solution while using polyimide resins different in dissolution rate.